For adjusting a pitch angle of rotor blades of a helicopter, known hitherto is such a type of device as coupling a pitch link 63, as shown in FIG. 12, through spherical bearings provided on opposite end portions with a point of the nose portion of a pitch horn 62 of a pitch housing 61 provided projectingly from an end portion coming out of a pitch axis, which is fixed to a rotor blade 55 at its mounting end portion on a hub and supported on the hub 60 rotatably round the pitch axis, and a point corresponding thereto on a rotating ring 8 of a swash plate thus adjusting the length to adjust a pitch angle of the rotor blade 55.
FIG. 13 is a diagram representing a construction of one example of the prior art pitch link 63. As shown in this figure, the pitch link 63 comprises, a pair of end rings 66 each having a spherical bearing 64 on one end and a thread running in the direction counter to each other on the other end into the end rings 66 are screwed female screws counter to each other which are provided on opposite ends of a barrel rod 67. In order to adjust the length of the pitch link, a normal practice is that the barrel rod 67 is turned to perform fine adjustment of the length between the spherical bearings on both ends, and a lock nut 68 screwed onto the screw 65 of each end ring 66 is tightened and thus pressed against an end surface of the barrel rod 67, to fix the end ring 66 securely so as not to be loose against the barrel rod.
Meanwhile, vibrations of a helicopter are caused by an unbalance of a rotor, and, if such unbalance is excessive, then not only discomfort of passengers but also structural fatigue may unexpectedly be caused.
A decrease in vibrations of a helicopter has been attemped hitherto such that a deviation in path of the tip of rotor blades is measured on a strobotracker while flying, or a vibration level is measured on a vibrosensor, and based on the data obtained therefrom, when a rotation of the rotor is stopped after landing by a regulation indexed by chart or analyzer for the pitch link, a length of the pitch link is adjusted manually, and then the vibration level is ensured through reflying.
Due to a difference in flying conditions, a dispersion of the pitch link adjustment or other factors, flying is required 5 or 6 times normally until the vibration level settles at a required value or below, and in addition an imperfect adjustment may result in most cases.
Requirements for vibrations of the helicopter have become of late severer and severer, and thus how to decrease vibrations of the helicopter is regarded as one of the grave issues in the future. To satisfy such requirements according to the conventional method, the number of times of flights inevitably increases more than hitherto, and yet there is a limit for the fine adjustment due to a dispersion of the pitch link adjustment and so forth.
The dispersion of the pitch link adjustment may also be caused by a construction of the pitch link in which the length will be adjusted by a screw coupling of an end ring and a barrel rod. In the screw coupling, there is a play present between a male screw and a female screw, and hence when fixing each end ring by clamping the lock nut, a position where the end ring is fixed is shifted, even though it is minute, by a frictional force working between the screw surface and the lock nut surface according to such clamping, therefore it is difficult to adjust precisely the length of the pitch link, thus leaving a dispersion.
As mentioned hereinabove, 5 to 6 times of flights are required until the vibration level is decreased to a desired value or below, and still an adjustment will not satisfactorily be realized in most cases, however, a dispersion of the aforesaid pitch link adjustment is assumed to be one of substantial factors thereof.